1. Field of the Invention
The present invention relates to an objective lens suitable for use with an optical head of an optical disk drive such as a magneto-optical disk recording and reproducing apparatus or an optical disk reproducing apparatus.
2. Related Background Art
An objective lens for focusing a laser beam to a recording surface of an optical disk in a record mode and condensing a laser beam scattered from the recording surface of the optical disk in a reproduction mode is mounted in an optical head of an optical disk drive such as a magneto-optical disk recording and reproducing apparatus or an optical disk reproducing apparatus. Such an objective lens is required to have the following properties.
(1) Light in weight. In order to increase a response speed of focusing and tracking, it is necessary to drive the objective lens at a high speed. To this end, it is necessary to reduce the weight of the objective lens to reduce a load to a lens actuator. PA1 (2) Small chromatic aberration. When information is to be recorded on a magneto-optical disk or write-after type optical disk, it is necessary to pulsively drive a semiconductor laser as a light source by a large amplitude from a low power to a high power. As the power changes, a wavelength of the semiconductor laser also changes. Thus, if the lens has a chromatic aberration, a focus point is shifted and a size of a spot of the laser beam on the recording surface of the optical disk increases. If the shift of the focus point is large, high quality recording is not attained. In a mark length recording method which permits higher density recording, a severe restriction for the chromatic aberration is required. PA1 (3) Large numeral aperture (N.A.) of the lens. In order to attain high density recording, it is necessary to converge a spot diameter of the laser beam on the recording surface of the optical disk to approximately 1 .mu.m. The numerical aperture required in a conventional lens is approximately 0.5, but as the high density is more and more required, the numerical aperture of approximately 0.55 is now required. PA1 (1) .nu..sub.1 &gt;70 PA1 (2) n.sub.1 &gt;1.55 PA1 (3) .nu..sub.1 -.nu..sub.2 &gt;45 PA1 (4) 1.1&lt;{-r.sub.2 /(f.multidot.NA)}&lt;1.5
In order to achieve those properties, a two-piece lens system which utilizes an aspherical surface has been proposed. (For example, Japanese Laid-Open Patent Application Nos. 1-296210 and 3-12618). Those known two-piece lens systems comprise a combination of a convex lens made of a low dispersion glass and a concave lens made of high dispersion glass.
However, the prior art objective lenses whose lens data has been disclosed exhibit the numerical aperture of 0.5, which is not sufficiently large. As to the correction for the chromatic aberration, the prior art objective lenses doe not provide sufficient correction to the mark length recording method which permits the higher density recording. One of the reasons therefor is that an Abbe's number of the convex lens and the concave lens is improper. Further, where a sufficient correction for the chromatic aberration is to be made, a radius of curvature on a joint surface is too small because of a diffraction index of the convex lens is small, and a thickness of the lens cannot be sufficiently large. On the other hand, if the thickness of the lens is sufficiently large, the numerical aperture of the lens is reduced.
Thus, in the prior art objective lens for the optical disk drive, it is difficult to attain a large numerical aperture while the chromatic aberration is sufficiently suppressed.